1. Field of the Invention
The present invention relates to a photographing system with a flash light device particularly a computer flash light device, which enables photography over a greatly expanded range of photographic light situations by controlling the diaphragm value in corespondence to the distance to an object.
2. Description of the Prior Art:
Of the flash photographic exposure control systems there have been two types one of which is so-called "Flash Auto" system. Said "Flash Auto" system regulates the diaphragm aperture valve by factoring the distance value from the camera to a subject into a given guide number. The other type is so-called "Computer Flash Light Device" system in which while the diaphragm aperture is fixed at a given value corresponding to the sensitivity of the used film, the duration of emission of the flash light is adjusted in accordance with the photo-electrically sensed distance to evolve a correct flash exposure value. It follows that the conventional exposure control system in the flash mode have functioned to regulate only one of the two parameters of diaphragm aperture and flash illumination in correspondence with the camera-to-subject distance. While the other parameter is made fixed at a value predetermined independently of the distance. Therefore, when a computer flash light device is used, an accurate exposure can be obtained only within a narrow range of object distance.
For a better understanding of the disadvantages of the prior art flash photography, the following description is made by referring to FIGS. 1A and 1B. In general, exposure values relating to functions of these four variable parameters. Namely, diaphragm aperture Av, camera-to-subject distance Dv, the amount of flash light emitted from flash device Pv and film speed Sv may be determined by applying the following formula in Apex terms to a given photographic situation. EQU Av+Dv=Pv+Sv=Gv (1)
wherein Gv is the flash guide number corresponding to the used flash source.
With "Flash Auto" system, this formula becomes: EQU Av=Gv-Dv (2)
The required value of diaphragm aperture is taken at a value obtained by dividing the guide number by the camera-to-subject distance since the parameter values are all expressed in logarithmic form. Now assuming that the given guide number is 22 at a film speed of ASA 100, and the range of diaphragm values available in a given camera objective is from 2.8 to 16 inclusive. Then the diaphragm aperture must be adjusted in accordance with the distance as shown by a solid line labeled A100 in FIG. 1A(a) wherein the ordinate represents the diaphragm value in f/step scale if representing F number) and the abscissa represents the distance in square ratio scale of meter. As the film sensitivity changes from ASA 100 to a lower or higher value of ASA 25 or ASA 400 respectively, the distance range over which reliable and accurate exposure values can be derived is shifted toward shorter and longer distances with the upper limits reaching 4 or 16 meters and with the lower limits being set at 0.7 and 2.8 meters respectively as shown by the dot-and-dash lines labelled A25 and A400.
The disadvantages of the "Flash Auto" system are that, since the total amount of flash light emitted for one actuation of the flash unit is always maintained constant and the total amount of flash light is constant irrespective of the distance.
Supposing that the amount of flash light corresponds to a guide number of 22 for a film of ASA 100, and the diaphragm aperture range of the taking range is set from 2.8 to 16, the diaphragm must be controlled to the value as shown by A100 in FIG. 1A(a) when a film of ASA 100 is used, and the photographable distance is limited to a range of 1.4 to 8 meters. Also when a film of lower sensitivity such as ASA 25 or higher sensitivity such as ASA 400, the diaphragm must be controlled as shown by A25 or A400, and the distance is limited to a range of 0.7 to 4 meters and a range of 2.8 to 16 meters respectively.
In case of the conventional flash auto system mentioned above, the amount of flash light is constant so that the range of the photographable distance for a film of a specific sensitivity is limited and narrow due to the adjustable range of the diaphragm, and also in this conventional system the diaphragm is changed just as the changed amount of film sensitivity so that it is not possible to photograph a subject at a short distance with a high-sensitivity film although it is possible to photograph a subject at a long distance.
For enabling the short distance photographing with a high-sensitivity film, a flash device of a small capacity of flash light may be used, but when a low-sensitivity film is used the problem that the long-distance photographing is impossible is not solved.
To eliminate the above-mentioned drawbacks of the "Flash Auto" system, a number of flash units of different light energy may be selectively used. As another example, use may be made of a single flash unit of which the flash light energy is manually changeable. In any case, the utility of the exposure control system is prejudiced to a large extent.
The disadvantages of the "Computer Flash Light Device" system according to the prior art will now be explained in detail by reference to FIG. 1B. When the distance is changed by one graduation, the amount of flash light emitted is changed by one step. In other words, when the distance is increased twice, the amount of flash light emitted is increased twice. With a flash unit having two discrete light levels labelled PH and PL as shown in FIG. 1B(b), when the used film has a speed of ASA 100 and the light adjustment characteristic of the flash unit is set at PH, the required value of diaphragm aperture is f/5.6 independently of the distance as shown in FIG. 1B(a) to effect a correct flash exposure. In this case, therefore, since the maximum amount of flash energy available is 22 at ASA 100, the reliable and accurate exposure control is made possible until 4 meters. When the light adjustment characteristic is changed to PL with the film speed unchanged from ASA 100, the diaphragm value is changed from f/5.6 to f/2.8 to derive a correct exposure value until 8 meters. When ASA 400 is used, the diaphragm value is changed by two steps from the position for ASA 100, and the upper limit of the distance range is 4 or 8 meters at AH400 or AL400 respectively. In other words, when the flash light adjustment characteristic is set at PH and the film of ASA 400 is used, the diaphragm value is required to be AH400 (F11) to effect a correct flash exposure. In this case, therefore, the exposure can be made for the subject at a maximum distance of 4 meters. When the adjustment characteristic is set at PL and the film of ASA 400 is used, the diaphragm value is required to be AL400 (F5.6) to effect a correct exposure. In this case, the upper limit of the distance range is 8 meters. With ASA 25, the flash light adjustment characteristic is set at PH and the diaphragm value is set at the fully open position for F2.8, the upper limit of the distance range is 4 meters as shown by AH25. But when set at PL, the diaphragm value must be adjusted to F1.4. Since the maximum possible diaphragm value is 2.8, it is impossible to derive a correct exposure value.
In other words, the computer flash light device varies only the diaphragm value depending upon the sensitivity of the used film. Therefore, when the light adjustment characteristic of PH is selected, the use of ASA 100 film leads to the setting the diaphragm value at 5.6 with the upper limit of the distance range being 4 meters, while the use of ASA 400 film leads to the setting the diaphragm value at 11 with the upper limit of the distance range being 4 meters also. For this reason, in the compute flash light device, even when the film speed is increased, the range of distances suited for correct exposure values can not be extended.
As is evident from the foregoing, the conventional computer flash light device does not make it possible to photograph subjects at increased distance (for example, compare AL100 with AL400). On the other hand, when a slow film is used, because of the limitation of the full open aperture value, it becomes impossible to photograph subjects at the increased distances and also with a shallow depth of field.
The above-mentioned drawbacks of the conventional exposure control systems are due to the limitation of the parameter which is varied with distance to either one of the diaphragm value and the amount of flash light emitted and also to the limitation of the parameter which is varied with film speed to the diaphragm aperture only.